Re: [PATCH] mm: Fix serialization adding transparent huge pages to page cache

[Jan Kara <jack@suse.cz>]

On Mon 20-06-22 19:05:36, Alistair Popple wrote:
> Commit 793917d997df ("mm/readahead: Add large folio readahead")
> introduced support for using large folios for filebacked pages if the
> filesystem supports it.
> 
> page_cache_ra_order() was introduced to allocate and add these large
> folios to the page cache. However adding pages to the page cache should
> be serialized against truncation and hole punching by taking
> invalidate_lock. Not doing so can lead to data races resulting in stale
> data getting added to the page cache and marked up-to-date. See commit
> 730633f0b7f9 ("mm: Protect operations adding pages to page cache with
> invalidate_lock") for more details.
> 
> This issue was found by inspection but a testcase revealed it was
> possible to observe in practice on XFS. Fix this by taking
> invalidate_lock in page_cache_ra_order(), to mirror what is done for the
> non-thp case in page_cache_ra_unbounded().
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> Fixes: 793917d997df ("mm/readahead: Add large folio readahead")

Thanks for catching this! Your fix looks good to me so feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza


> ---
>  mm/readahead.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 4a60cdb64262..38635af5bab7 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -508,6 +508,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
>  			new_order--;
>  	}
>  
> +	filemap_invalidate_lock_shared(mapping);
>  	while (index <= limit) {
>  		unsigned int order = new_order;
>  
> @@ -534,6 +535,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
>  	}
>  
>  	read_pages(ractl);
> +	filemap_invalidate_unlock_shared(mapping);
>  
>  	/*
>  	 * If there were already pages in the page cache, then we may have
> -- 
> 2.35.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR